Hearths and furnaces of the prior art which are generally utilized for the purposes mentioned above can be classified as stationary furnaces, rotary kilns, gravity rotationally discharging hearths, and the like.
Generally, more than one unit is necessary to accomplish the various step of a metallurgical ore-treating operation. Thus, for example, separate chambers are employed if the material to be treated requires, say, a pre-heating operation to drive off water and moisture prior to roasting, or if a predetermined cooling step is demanded after roasting, or if a number of different, temperature-dependent operations are to be carried out.
Furthermore, the same apparatus may be desired for handling materials which have inherently different characteristics. For example, some materials like perlite, require very gentle handling, because, being composed principally of silica, it cannot be tumbled in a rotary kiln and risk disintegration of its unique structure. Other materials possess varying amounts of water of hydration and necessitate stage-like handling in order to prevent bursting of particles.
The necessity, therefore, for a single, versatile apparatus for multi-purpose employment has long been felt by the industry, especially the metallurgical industry. Specifically needed is a hearth which is suitable for treating the most diversified materials, operates on a continuous basis, is capable of a high thermal efficiency in the transfer of heat from the heat source to the material, incorporates within a continuous operation a number of variable procedural steps, has a minimum of material losses, requires a minimum of operating labor, replacement parts and shut-downs, and is suitable to be used with varied feed rates, retention time and recovery rates.